The present invention relates to a method of preparing zeolite crystals with a determined pore system.
Zeolites find widespread use as heterogeneous catalysts. In many applications it is a problem that zeolites only have micropores, since this imposes diffusion limitations on the reaction rate. Several attempts to provide zeolitic crystals with a mesoporous system have been made in the past. One possibility is to lower the crystal size of the zeolite, the void in between the individual crystals forming the mesopores (intercrystalline pores), (Madsen C. Jacobsen, C. J. H., Chem.Comm. (1999) 673) and another possibility is to use mesoporous MCM-41 type materials (e.g. Beck, J. S. et al. J.Am.Chem.Soc. 114 (1992) 10832), although they only exhibit a strict two-dimensional order and consequently do not posses the same acidity as zeolites with three-dimensional order.
This invention provides a method for preparing zeolite crystals with a tuneable pore system of zeolite crystals. The method involves crystallisation of the zeolite within and on surface of a matrix consisting of particles of which a majority is in the size range of 10-500 nm. In this way it is possible for the zeolite to grow into large single crystals encapsulating part of the matrix particles. The matrix is impregnated both inside and on the surface of the matrix, if the amount of impregnation solution (zeolite gel) applied is higher than the pore volume characteristic of the matrix (excess of gel compared to incipient wetness).
When using e.g. carbon as the matrix, the zeolite crystals can be isolated by removal of the carbon either by a controlled combustion or by hydrogenation. Other matrices can be used and removed e.g. by selective dissolution or by hydrolysis etc. The matrix is preferably inert and stable under zeolite synthesis conditions and exhibits appropriate pore morphology. By removing the matrix from the large single crystals, mesopores are created inside the individual large crystals, i.e. intracrystalline mesopores.
Large zeolite crystals with intracrystalline mesopores are easily separated from the synthesis medium by means of filtration, whereas micro zeolite crystals (being able to create intercrystalline mesopores) require ultracentrifugation in order to be separated from the synthesis medium. Another advantage of large single zeolite crystals is that the hydrothermal stability of the zeolite is increased.
It is preferred to remove the matrix to an extent, which does not hinder transport of reactants and products in the mesopores. However, complete removal of the matrix is not required. Preferably, the remaining amount of matrix material inside the zeolite crystals is as low as possible.